1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
The coverage area of a wireless communication system is typically divided into a number of geographic areas that are conventionally referred to as cells. The coverage area of each cell in the wireless network is limited by the propagation loss of radio signals transmitted by base stations that provide coverage to the cell. Thus, the coverage area of each cell is determined by the location and the transmit power of the base station, as well as the geography of the cell in the location of any interfering objects. For example, the coverage area of a cell may be reduced if a building or a mountain is present near the base station. The boundaries of the cells are not rigidly defined and may vary with time. Thus, coverage areas may overlap such that multiple base stations may provide coverage to the overlapping regions, although the strength of the signal provided within the overlapping regions may be different for the different base stations.
Mobile units located in the coverage area of a cell may access the wireless communications system by establishing a wireless communication link, often referred to as an air interface, with a base station associated with the cell. Since the cells may overlap and multiple base stations may provide wireless connectivity to each mobile unit, mobile units typically select the base station (and/or the associated cell) that provides the best connection to the wireless communication network. The selected base station may then become the serving base station for the mobile unit and the mobile unit may form a wireless communication link (or air interface) with the serving base station.
The serving base station may be selected by comparing a variety of parameters including the quality of service that may be provided over the air interface, bit error rates associated with signals transmitted over the air interface, signal strengths, interference, and the like. The connection parameters associated with each base station are typically determined using measurements of a pilot signal broadcast by each base station and received at the mobile unit. For example, the pilot signal may be transmitted on a pilot channel such as an unmodulated, direct sequence spread spectrum signal transmitted by base stations that operate according to Code Division Multiple Access (CDMA) protocols. The pilot signal may provide a phase or timing reference for coherent demodulation, a reference for signal strength or quality of service comparisons between base stations, and the like.
A list of the sectors that neighbor the current sector, conventionally referred to as a neighbor list, is broadcast to mobile units within the coverage area of the wireless communication network. The mobile unit may then use the neighbor list to select pilot signals that should be monitored to decide which base station should act as the serving base station. For example, a first base station may be providing wireless connectivity to the mobile unit in a first sector. The neighbor list for the first sector may indicate that second and third sectors are adjacent the first sector. The mobile unit may monitor pilot signals transmitted by one or more base stations associated with the first, second, and third sectors. If the mobile unit roams into one of the neighboring sectors, the monitored pilot signals may indicate which sector would provide superior connectivity to the mobile unit and the mobile unit may hand off to this sector.
The mobile unit relies upon the wireless communication system to provide a neighbor list that accurately reflects the sectors that are neighbors of the (serving) sector that currently includes the mobile unit. If the neighbor list contains incorrect information, a mobile unit may be misled into monitoring incorrect or nonexistent pilot signals. Consequently, an inaccurate neighbor list may cause access failure or call drops due to the inconsistency between the actual context of the mobile unit and the context indicated in the neighbor list.
Neighbor lists are conventionally generated and/or populated empirically using measurements performed at various locations within the coverage area of the wireless communication system. For example, test engineers may take test drives through the coverage area of the wireless communication system and use special equipment to scan the entire spectrum in the neighboring areas. These measurements may be used to identify the pilot signals that are likely to be most visible to mobile units located in different test locations within the sector/cell coverage. The identified pilots are manually populated into the system configuration, one sector-carrier a time. Each sector-carrier can be put in-service only after the whole testing process and the associated configuration are completed and verified.
Conventional techniques for generating and/or populating the neighbor list are time-intensive, costly, and prone to error. Any human mistake in performing or recording the measurements, as well as in populating the neighbor list and configuring a wireless communication system, may cause an incorrect or inaccurate neighbor list. The neighbor list may also be incorrect or inaccurate if the testing engineer bypasses or misses any areas within the coverage area. Furthermore, the neighbor lists determined using conventional techniques are static and not able to respond to time-dependent variations in pilot signal strengths or cell boundaries. These inaccuracies may degrade performance of the system and in some cases may even cause significant outages.